Mount root from SD card?
krassi at bulinfo.net
Thu Jan 22 06:46:42 PST 2009
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I have added a loop to read (256 times) the receive register (MCI_RDR)
right before DMA setup for reading. Here is the result:
Bernd Walter wrote:
> On Thu, Jan 22, 2009 at 03:22:28PM +0200, Alexander Motin wrote:
>> Bernd Walter wrote:
>>> On Wed, Jan 21, 2009 at 12:28:42PM -0700, M. Warner Losh wrote:
>>> Havn't read everything, so maybe I writing nonsense in respect to
>>> this problem.
>>> The multiblock read trouble is a problem in the MMC design.
>>> The first read always works fine, but you need to stop the transfer
>>> and an exact time, otherwise it starts reading the next block.
>>> The first bytes of the next block then stucks in the DMA fifo and
>>> the next read then starts with the stuck uint32.
>>> If you see broken reads like this then I would assume the previous
>>> command was problematic.
>>> The official workaround to get multiblock reading is to reset the MMC
>>> and clean the fifo after each multiblock command, but I would assume
>>> it is enough to just read the data register a few time before setting
>>> up the DMA for the new request.
>> Haven't look deep at that arm controller operation, but standard SD host
>> has special counter register which terminates DMA transfer after
>> specified amount of blocks. sdhci driver even uses it's Auth-CMD12
>> feature, where controller even sends STOP command to the card by itself.
> Yes - and this is broken in the Atmel design.
> This is not the only nasty bug that Atmel has left int he chip.
> You need to issue the STOP command at the absolut right timing, which
> is more or less impossible.
> The design puts every 32bit word in a receive register, which has a small
> You can setup a DMA enginge to pull the words from the register into RAM.
> Now what happens is that if you issue the STOP too late it starts reading
> the next block and then stops - the card can stop at every location, so
> there is no problem about this, but now you have the first words in the
> receive fifo.
> The DMA doesn't work anymore, because it was setup with a limited number
> of words to pull, so the additional words are kept in the register.
> Once you start reading the next block and setup the DMA it pulls the
> stuck words first.
> It that case it looks as if the timing is one word too late therefor
> you get 4 additonal bytes after each transaction.
> It shouldn't be a problem to read the receive register without data in
> it, so 4 or 5 dummy reads befor DMA setup should be enough.
> IIRC the fifo can only hold up to 3 words.
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